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Article

Fretting-Corrosion Behavior of Stellite 6 Overlay Welded on 304 Stainless Steel in Simulated PWR Water Environment

by
Yuanbin Gui
*,
Chengtao Li
*,
Zhaoguang Zhu
,
Sunwu Xu
,
Bin Yang
,
Qianwu Li
,
Jing Wan
and
Shugang Zhang
Suzhou Nuclear Power Research Institute, Suzhou 215004, China
*
Authors to whom correspondence should be addressed.
Lubricants 2026, 14(4), 149; https://doi.org/10.3390/lubricants14040149
Submission received: 28 February 2026 / Revised: 20 March 2026 / Accepted: 21 March 2026 / Published: 31 March 2026
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Abstract

The fretting-corrosion behavior of a Stellite 6 cobalt-based overlay welded to 304 stainless steel was investigated in simulated high-temperature, high-pressure PWR water. Three material pairings were examined: Stellite 6/Stellite 6 (C-C), Stellite 6/304 stainless steel (C-S), and 304 stainless steel/304 stainless steel (S-S). Wear behavior was evaluated in terms of mass loss, surface morphology, surface chemistry, friction evolution, and subsurface deformation. The results show that material pairing strongly affects friction stability and damage evolution during fretting corrosion. The C-C contact exhibited a relatively stable coefficient of friction and continuous wear morphology, with damage dominated by plastic deformation. In contrast, the C-S and S-S contacts exhibited stronger wear–corrosion interaction, characterized by debris accumulation, oxide film instability, and fluctuating friction behavior. Despite the same oxide species being observed in different contact pairs, their distribution and stability varied greatly, which resulted in different modes of damage. EBSD analysis showed that fretting energy in the C-C contact was mainly accommodated by plastic strain in the near-surface region, whereas deformation in the C-S and S-S contacts was more localized and discontinuous. These results indicate that oxide film stability and subsurface strain distribution jointly control friction behavior and fretting-corrosion damage under different material pairings.

1. Introduction

During the operation of pressurized water reactors (PWRs), turbulence, pressure fluctuations and local velocity variations of the primary coolant in complex flow channels can induce flow-induced vibration (FIV) on component surfaces [1,2]. At critical locations—steam generator tube-anti-vibration bar contacts, in-core component mating interfaces and valve sealing surfaces—such vibration typically takes the form of small-amplitude, high-frequency oscillatory motion at the contact interface, leading to fretting-corrosion damage in high-temperature water environments [2,3,4,5,6,7]. Under high-temperature water environments, mechanical wear is often coupled with corrosion reactions, which accelerates damage evolution and may ultimately result in fretting-corrosion failure, thereby threatening the structural integrity of nuclear power components [6,7,8,9].
Cobalt-based overlay hardfacing is widely employed to improve the wear resistance of these critical components [10,11]; among them, Stellite-type alloys, comprising a γ-Co solid solution reinforced by Cr-rich carbides (primarily M7C3), combine high wear resistance and corrosion stability and are extensively used for valve seats and mating surfaces in reactor internals [12,13]. Prior work on cobalt-based alloys has largely addressed sliding wear mechanisms and related subsurface responses [12,13,14,15,16], while nuclear-field fretting studies have mainly focused on fuel assembly spacer grids and steam generator tubes [17,18]. Systematic investigations of fretting-corrosion behavior and damage mechanisms for hardfacing systems on valve and in-core surfaces—particularly under simulated PWR primary-circuit conditions (320 °C, 13 MPa)—remain limited [18,19,20].
This study therefore examines the fretting-corrosion behavior of a Stellite 6 overlay on 304 stainless steel under simulated PWR conditions. Three representative pairings were tested—overlay/overlay (C-C), overlay/304 (C-S) and 304/304 (S-S)—and mass loss, 3D surface morphology, Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and Electron Backscatter Diffraction (EBSD) were combined to elucidate surface and subsurface damage evolution, providing experimental support for evaluating hardfacing performance and selecting materials for nuclear applications.

2. Materials and Methods

2.1. Materials

Stellite 6 cobalt-based overlay and 304 stainless steel were selected for the fretting-corrosion study. The Stellite 6 overlay was deposited on 304 stainless steel substrates by manual TIG welding. Four overlay passes produced a total overlay thickness of approximately 7 mm. High-purity argon served as the welding shielding gas. The deposited overlays showed good surface appearance and dense metallurgical bonding to the substrate; no obvious welding defects (pores, cracks or slag inclusions) were observed.
Figure 1 shows the metallographic microstructure of the Stellite 6 overlay. The microstructure comprises a γ-Co solid-solution matrix with uniformly dispersed Cr7C3-type carbides; the microstructure is continuous and homogeneous without macroscopic segregation or defects, indicating good metallurgical quality. These features are consistent with typical Stellite 6 overlays reported in the literature [21].
The chemical compositions of the overlay and the 304 substrate were measured by optical emission spectroscopy. Results are listed in Table 1 and comply with the relevant ranges of the RCC-M standard [22].

2.2. Specimen Preparation and Grouping

Fretting-corrosion tests were performed using a flat-on-flat contact configuration, in which a rectangular boss was machined on the counter specimen to define the nominal contact area. The fixed specimen measured 24 mm × 10 mm × 3 mm, whereas the counter specimen measured 34 mm × 10 mm × 2 mm. The boss dimensions were 5 mm × 5 mm × 2 mm, with a fillet radius of 1 mm at the edges to reduce local stress concentration. A schematic diagram of the sample structure is shown in Figure 2a.
In all tests, the boss specimen served as the counter specimen and was subjected to cyclic tangential displacement, while the flat specimen was fixed. This configuration ensured that fretting motion was confined to the boss region and that the flat specimen primarily acted as the load-bearing surface. A schematic of the specimen assembly is shown in Figure 2b.
Three representative material pairings were investigated: overlay/overlay (C-C), overlay/304 stainless steel (C-S), and 304 stainless steel/304 stainless steel (S-S). For each pairing, both boss (counter specimen) and flat (fixed specimen) surfaces were prepared using the corresponding materials. This design enabled a systematic comparison of fretting-corrosion behavior between the boss and flat surfaces under identical contact and environmental conditions. The grouping and numbering of the samples are shown in Table 2.

2.3. Fretting-Corrosion Test Conditions

Prior to the fretting tests, the contact surfaces were sequentially ground using SiC abrasive papers with grit sizes of 600, 1000, 2000, 3000, 5000, and 7000, followed by final polishing using a 1 μm diamond suspension to obtain a mirror-like surface finish. Such a polishing procedure is known to produce a fine surface finish with a roughness typically below Ra 0.1 μm [23]. Both contact surfaces in each material pair were prepared using the same procedure to ensure consistent initial surface conditions.
Fretting-corrosion tests were carried out using a Bruker (Baio) FMS-0.5 fretting-corrosion tester (Bruker, Billerica, MA, USA). A normal load of 60 N was applied, with a vibration frequency of 25 Hz and a displacement amplitude of ±60 μm. The total number of fretting cycles was 2 × 105. The fretting parameters used in this study were selected to simulate typical vibration-induced contact conditions that may occur in nuclear power plant components operating in a pressurized water reactor (PWR) environment. The normal load, displacement amplitude, and frequency were selected based on values commonly reported for fretting corrosion of structural materials in simulated reactor primary water environments [24].
All tests were conducted in a simulated PWR primary-circuit water at 320 °C and 13 MPa. The test solution contained 1200 ppm B3+ (as H3BO3) and 2.2 ppm Li+ (as LiOH). Prior to testing, high-purity nitrogen was purged into the system for 1.5 h to minimize the dissolved oxygen content. All material pairings were tested under identical conditions to ensure result comparability.

2.4. Characterization Methods

After the fretting-corrosion tests, the specimens were ultrasonically cleaned in anhydrous acetone for 5 min to remove loosely adhered debris. No additional chemical cleaning was applied in order to avoid removing compacted oxide layers or tribolayers formed during fretting. The samples were subsequently dried and weighed using an analytical balance to determine the mass loss. The macroscopic and microscopic morphologies of the fretting scars were examined by scanning electron microscopy (SEM) (Helios G4 UX, Thermo Fisher, Waltham, MA, USA). Elemental distributions on the worn surfaces were analyzed using energy-dispersive spectroscopy (EDS) (Helios G4 UX, Thermo Fisher, Waltham, MA, USA).
X-ray photoelectron spectroscopy (XPS) (PHI5000 VersaProbe III, ULVAC-PHI, Chigasaki, Japan) was used in order to establish the chemical states of surface oxides and wear materials. Besides that, cross sections of typical fretting scars were fabricated in order to perform electron backscatter diffraction (EBSD) (Helios G4 UX, Thermo Fisher, Waltham, MA, USA) to identify changes in crystallographic orientation and localized plastic strain build-up in near-surface area and provide some microstructural information on fretting-corrosion mechanisms.

2.5. Data Reliability and Fretting Regime Identification

For each material combination, duplicate tests were conducted. Duplicate tests were conducted for each material combination, and the results showed good consistency in friction response, mass loss, and wear morphology. Together with the clear differences observed among the material pairings, this supports the reliability of the present analysis. The fretting scars exhibited characteristic material removal features associated with continuous sliding at the contact interface, suggesting that the fretting conditions in this study were predominantly within the gross slip regime. Under such conditions, both mechanical wear and tribo-oxidation are expected to contribute to the damage evolution.
Based on these considerations, the fretting-corrosion behavior of different material combinations was analyzed in terms of macroscopic damage characteristics, surface morphology, chemical composition, and subsurface microstructural evolution.

3. Results and Discussion

3.1. Macroscopic Fretting Behavior and Mass Loss Characteristics

The macroscopic fretting behavior was first evaluated based on mass loss and surface topography. Table 3 summarizes the mass changes of the specimens before and after fret-ting-corrosion tests for different material pairings. Figure 3 presents the macroscopic appearances of the flat specimens together with the corresponding three-dimensional (3D) surface profiles of the fretting scars. For all contact pairs, the protrusion (boss) specimens consistently exhibited higher mass loss than the corresponding flat specimens. This difference does not indicate a change in the dominant wear mechanism, but rather reflects the influence of contact geometry on wear-debris behavior.
Due to the spatially confined contact region of the protrusion specimens, wear debris is less readily expelled from the interface and tends to accumulate within the contact zone. The retained debris can participate in subsequent fretting cycles, promoting repeated rupture of surface oxide or passive films and accelerating fretting-corrosion damage accumulation [14,15]. In contrast, flat specimens experience more uniform contact conditions and exhibit more continuous fretting scars, which better reflect the intrinsic fretting-corrosion behavior of the materials. Therefore, flat specimens were selected as representative samples for subsequent analyses of surface morphology, microstructural evolution, and surface chemical states to minimize the influence of contact geometry.
Under the C-C contact condition, all specimens exhibited relatively low mass loss, ranging from 0.12 to 0.50 mg. The fretting scars were localized with well-defined boundaries and limited lateral extension, and no pronounced edge tearing or large-scale material accumulation was observed. The 3D surface profiles revealed small height variations and a concentrated wear-depth distribution, indicating a relatively stable fretting-corrosion process in which material removal was mainly confined to the near-surface region.
When 304 stainless steel was introduced into the contact pair (C-S), the mass loss of the Stellite 6 boss specimens increased markedly to 0.88–1.22 mg. Correspondingly, the fretting scars exhibited irregular boundaries, while the 3D profiles showed the coexistence of depressions and localized protrusions with a highly non-uniform wear-depth distribution. These features suggest that under dissimilar material contact conditions, differences in interfacial mechanical response and surface chemical behavior lead to a more complex fretting-corrosion process. Repeated rupture and regeneration of surface protective layers on the hardfacing material therefore contribute to the aggravated damage [25,26,27]. It is noted that slight “negative mass loss” was observed for some 304 stainless steel flat specimens in the C-S pair. This phenomenon does not represent actual material gain, but results from the accumulation of wear debris and fretting-corrosion products within the scar, which is favored by the small displacement amplitude and limited debris expulsion during fretting [28].
For the S-S contact pair, the overall mass loss remained low (≤0.14 mg). However, macroscopic observations and 3D surface profiles revealed dispersed height distributions and relatively high apparent surface relief within the scars. This indicates that under same-material stainless steel contact conditions, the formation, rupture, and reformation of surface products dominate scar evolution. Consequently, the 3D topographical features did not exhibit a simple linear correlation with macroscopic mass loss [27,28,29,30,31,32].
Overall, combining mass loss data with 3D surface morphology analyses demonstrates that as the contact materials transition from the cobalt-based overlay to stainless steel, the fretting-corrosion behavior evolves from a relatively stable, mechanically dominated wear mode toward a more complex damage mode in which surface products actively participate and wear–corrosion interaction becomes more pronounced.

3.2. Microscopic Morphology of Fretting-Corrosion Scars

SEM micrographs of the fretting-corrosion scars on the flat specimens with various contact pairs are illustrated in Figure 4. The material combinations had different scar morphologies, which emphasizes the importance of contact pairing in the control of fretting-corrosion behavior.
In the case of the C-C contact condition, the wear scars on the cobalt-based overlay layer were fairly flat and had clearly marked borders. Low-magnification showed that there was only little and locally localized wear debris accumulation. Fine and dense scratches could be seen at high magnification in the direction of the fretting slide without any evident spallation pits or large scale surface damage. The mild severity of damage is also demonstrated by the good continuity of the wear scars, which suggests that the predominant mode of wear due to the fretting process is stable mechanical wear.
On the other hand, contact pairs with 304 stainless steel had significantly different morphologies. The C-S contact pair low-magnification SEM images revealed irregularities in surface coverage in the areas of the wear scars that partly hid the underlying wear characteristics. At high magnifications, discontinuous oxide layers were seen on the surface of the stainless steel and there was also local film rupture and mild delamination. Such indications suggest that the stainless steel surface experiences repeated breakdown and regeneration of the oxide film due to fretting, which causes a fretting-corrosion mechanism that is regulated by the mutual effects of mechanical wear and oxidation reactions.
These features became increasingly evident as the contact configuration shifted to the S-S pair. Wear scar area increased drastically and the surface morphology became extremely heterogeneous. At high magnification, SEM images revealed an increased surface roughness, the presence of discontinuous oxide product accumulation, and localized spallation. This indicates that the oxide film that forms on the surface of 304 stainless steel is poorly stable under fretting loading and does not transform into a coherent and compact protective coating.
The stainless steel specimens exhibited relatively large local wear depths in the three-dimensional surface profiles, whereas the corresponding macroscopic mass loss remained comparatively small. This apparent discrepancy is closely associated with the formation, accumulation, and dynamic evolution of wear debris and oxide products within the fretting scars. After the fretting tests, the specimens were ultrasonically cleaned in anhydrous acetone for 5 min prior to weighing in order to remove loosely attached debris. However, compacted oxide layers or tribolayers that formed during fretting may remain partially adhered to the surface. As a result, these surface covering layers may partly compensate for material removal during mass loss measurements, which is consistent with the macroscopic results discussed in Section 3.1 [33].

3.3. Surface Phase Analysis of Wear Scars

In order to explain the chemical roots underlying the dissimilarities in fretting-corrosion behavior between different contact pairs, EDS and XPS measurements were carried out on the wear scar areas. Figure 5 shows the EDS elemental distribution maps of the flat specimens after fretting tests, reflecting the elemental composition and spatial distribution in the near-surface regions of the wear scars.
For the C-C contact pair, the distributions of O, Cr, and Co within the wear scar were relatively uniform, and no obvious local enrichment or introduction of foreign elements was observed. This indicates that the wear products are mainly derived from the cobalt-based overlay itself, with limited material transfer or product accumulation during fretting. In contrast, the C-S and S-S contact pairs exhibited pronounced spatial heterogeneity in elemental distribution. In particular, O showed localized enrichment within the wear scars, while the distribution of Cr was relatively dispersed and did not strictly coincide with that of O. These results suggest that under heterogeneous or stainless steel self-mated contact conditions, the formation and accumulation of oxidation and wear products become highly non-uniform, reflecting the increased complexity of the fretting-corrosion process.
To further elucidate the chemical states of the surface films, XPS analyses were conducted. Figure 6a–c presents the XPS survey spectra of the wear scar surfaces for different contact pairs, while Figure 6d,e shows the high-resolution O1s and Cr2p spectra, respectively. For comparison purposes, the high-resolution spectra were normalized to their maximum intensities. The survey spectra revealed the presence of O and Cr signals on all wear scar surfaces, confirming that oxidation reactions universally occur under high-temperature and high-pressure fretting conditions.
For the C-C contact pair, the wear scar surface was mainly characterized by Co, Cr, and O signals, indicating that the oxide film is predominantly derived from the intrinsic components of the cobalt-based overlay alloy. In the C-S contact pair, EDS results indicate the involvement of Fe in material transfer and product formation within the wear scar region. However, Fe-related signals were not prominent in the XPS spectra of the outermost surface layer, suggesting that the surface oxide film remained dominated by Cr-enriched oxides. This implies a stratified or chemically heterogeneous oxide structure in which Fe-containing phases are likely distributed beneath the Cr-rich oxide film or intermittently covered by it [30,31].
The high-resolution O1s spectra showed similar peak positions for all contact pairs, without pronounced peak splitting or significant differences in peak shape. This indicates that the primary oxide species that formed under different fretting conditions are broadly comparable. Therefore, the differences in fretting-corrosion behavior among the contact pairs are not associated with fundamental changes in oxide type, but rather with variations in oxide film continuity, stability, and interfacial bonding.
Consistently, the Cr2p spectra for all contact pairs exhibited characteristic peaks corresponding to Cr2O3, confirming that Cr-enriched oxides constituted a major component of the surface oxide films. Compared with the C-C contact pair, the Cr oxide-related signals in the C-S and S-S contact pairs appeared more stable, suggesting an enhanced tendency for Cr surface enrichment during oxide film formation under heterogeneous and stainless steel self-mated contact conditions.
Although EDS detected significant Fe signals within the wear scars of the C-S and S-S contact pairs, XPS indicates that Fe was not dominant in the outermost surface layer. This discrepancy can be explained by the different probing depths of the two techniques. EDS collects compositional information from a depth of approximately 1–100 μm, whereas XPS is a surface-sensitive method with a probing depth of about 10 nm [34]. Therefore, EDS reflects signals from both the oxide layer and the underlying substrate, while XPS mainly characterizes the chemistry of the outermost surface.
The observed surface chemistry can also be interpreted in terms of the selective oxidation of stainless steel at elevated temperatures. Chromium has a higher affinity for oxygen than iron and preferentially forms thermodynamically stable Cr2O3 or Cr-rich spinel oxides. Consequently, the outer surface tends to become enriched in chromium oxides, whereas Fe-containing phases are more likely to remain in subsurface regions or are removed during fretting-induced oxide film rupture [35].
These results suggest that the oxide film that formed on the stainless steel surface during fretting corrosion may exhibit a layered or compositionally heterogeneous structure, consisting of a Cr-enriched outer layer and Fe-containing subsurface regions. Overall, the combined EDS and XPS analyses indicate that although similar oxide species form on different contact pairs, their distribution and stability differ significantly, which in turn affects oxide film integrity and fretting-corrosion behavior.
The XPS results provide information on the chemical composition of the outermost surface layer, which is critical for understanding the formation and stability of tribo-oxide films during fretting corrosion. Although the thickness of the oxide films was not quantitatively measured in this study, the combined SEM, EDS, and XPS analyses offer consistent evidence regarding the presence, composition, and spatial distribution of the oxide layers. These results are sufficient to support the interpretation of the tribo-oxidation behavior and its role in fretting damage evolution.

3.4. Subsurface Deformation and Strain Accumulation Behavior Based on EBSD

The kernel average misorientation (KAM) value reflects the degree of intragranular lattice distortion and is widely used as an indicator of plastic strain accumulation and dislocation activity in near-surface regions [25]. Figure 7 presents the EBSD inverse pole figure (IPF) maps and the corresponding KAM distributions of the wear scar regions on flat specimens under different fretting-corrosion contact conditions.
For the C-C contact pair, pronounced banded orientation distortion features were observed within the wear scar region, preferentially aligned along the fretting direction. In regularity, high-misorientation areas were continuously and densely mapped on the KAM maps, which indicate considerable cumulative plastic deformation of the near-surface layer. The given behavior can be attributed to a significant rise in the density of dislocations, where the deformation is mainly localized near the surface [16]. Such characteristics are well-correlated with the comparatively greater mass loss and the plowing plasticity of the wear scars, indicating that when the contact condition is C-C, fretting-corrosion loading easily leads to prolonged surface plastic deformation and microstructure reorganization [16,25].
On the other hand, the KAM distributions of the C-S and S-S contact pairs had lower-misorientation regions to dominate them, with only a few high-KAM signals that could be found close to some grain boundaries or in limited subsurface regions, which implies a significant inhibition of continuous plastic deformation. In the case of the S-S contact pair in particular, the high-misorientation zones were discrete, and some of the accumulated strain was found within grain interiors or subsurface zones. It indicates that stresses due to fretting are more probably accommodated by non-uniform deformation processes including microcrack initiation, debris detachment, and local oxide film rupture, instead of uniform surface plastic flow [30,31,32].
Overall, the EBSD-KAM analysis demonstrates that material pairing plays a critical role in governing the spatial distribution and accumulation mode of plastic strain during fretting corrosion. Highly concentrated near-surface plastic strain in the C-C contact pair contributes to wear evolution, whereas the localized and subsurface redistribution of deformation in the C-S and S-S contact pairs partially mitigates severe surface plastic damage. These crystallographic-scale observations provide strong microstructural support for the fretting-corrosion mechanisms inferred from the wear scar morphology and surface chemical analyses discussed in Section 3.1, Section 3.2 and Section 3.3.

3.5. Evolution of the Coefficient of Friction During Fretting Corrosion

Building on the differences in the wear morphology, surface chemistry, and subsurface deformation revealed in Section 3.1, Section 3.2, Section 3.3 and Section 3.4, the evolution of friction behavior provides an integrated macroscopic response to the underlying fretting-corrosion mechanisms associated with different material pairings. Figure 8 illustrates the evolution of the coefficient of friction (COF) as a function of cycle number for different material pairings during the fretting-corrosion tests. All contact pairs exhibited pronounced COF fluctuations in the initial stage, corresponding to the running-in regime dominated by asperity contact, initial debris generation, and rapid interfacial adjustment [32,36]. Notably, the initial COF peaks differed significantly among the material pairings, with the stainless steel-stainless steel (S-S) contact exhibiting the highest values, indicating poorer interfacial conformity and a more unstable initial contact condition [28,32].
With increasing cycle number, all contact pairs gradually evolved toward a quasi-steady friction regime; however, their frictional behaviors remained distinctly different. The cobalt-based overlay self-mated contact (C-C) maintained a consistently low COF with minimal fluctuation throughout the test, demonstrating excellent frictional stability. This behavior correlates well with the continuous wear scar morphology, the formation of a relatively simple and stable Co-Cr composite oxide film, and the highly concentrated near-surface plastic deformation revealed by EBSD analysis. Together, these characteristics indicate that a stable surface state combined with coordinated plastic deformation effectively suppresses frictional instability under fretting-corrosion conditions.
In contrast, the cobalt-based overlay against stainless steel contact (C-S) exhibited an intermediate COF level accompanied by periodic fluctuations, reflecting the dynamic evolution of the heterogeneous interface. Considering the coexistence of debris accumulation and surface damage within the wear scars, as well as the formation of mixed oxide and material transfer layers, the repeated rupture and reformation of the surface oxide film was identified as the primary origin of the observed friction oscillations.
In the case of the stainless steel self-mated contact (S-S), the COF was significantly high, and even in the quasi-steady regime, there were still friction spikes (as illustrated by the arrows in Figure 8) indicating significant frictional instability. Such behavior is supported by the data from the SEM, EDS, and EBSD above-mentioned, showing discontinuity of oxide films, diffuse subsurface strain distribution, and localized spalling. These findings imply that when S-S contact occurs, the increased wear-corrosion interaction prevents the development of an interfacial state, resulting in the ongoing instability of friction, which is consistent with previous reports that oxide regions that formed during high-temperature tribological contact can break up into abrasive particles and thereby modify both friction and wear behavior [37].

3.6. Wear–Corrosion Interaction Under Different Material Pairings

In order to further explain the phenomena of the fretting-corrosion behavior that was observed with various material combinations, the mechanical wear reactions and oxidation features of the surfaces were reviewed in conjunction with each other. There was a repeated rupture and regeneration of surface oxide films under the action of cyclic micro-slip in high-temperature and high-pressure PWR water conditions. Thus, the overall fretting-corrosion response can be seen as the result of a dynamic balance between mechanical material removal and oxide film development on the contact interface [38].
The relatively constant coefficient of friction, the constant wear scar morphology, and the localized near-surface plastic deformation of the C-C contact pair imply that the contact interface can accommodate the fretting energy mainly by accumulating plastic strain. Oxide films were created during the test, but their breaking and formation seemed to be relatively steady. The small amount of debris and no significant oxide spallation indicates that mechanical wear was the dominant factor of damage and that oxidation occurred in a minor and more uniform way.
On the other hand, the C-S and S-S contact pairs had a greater tendency toward higher friction variations and non-uniform wear patterns. The presence of oxide debris and transfer layer and locally damaged areas are indicated by the SEM and XPS findings. Together with the less concentrated and localized strain distribution in the subsurface as determined by EBSD, these characteristics suggest that the fretting-induced stresses are not relieved only through plastic deformation. On the contrary, repetitive oxide film fracture, entrapped debris, and interfacial material transfer lead to an unstable interface development. In such conditions, wear and oxidation processes become more closely connected, which causes increased wear–corrosion interactions and decreased stability of friction.
The comparative findings indicate that oxide film stability and subsurface strain tolerance have a combined influence on the fretting-corrosion behavior of various materials. In cases of concentrated and continuous plastic deformation, i.e., in the case of C-C contact, there is a tendency toward a relatively stable tribological condition at the interface. Conversely, when deformation is localized and oxide films become less stable, as seen in contacts containing stainless steel, the process of interfacial damage is more heterogeneous and friction variations remain. This interpretation offers a coherent rationale to explain the dissimilarities in mass loss, the friction development, surface chemistry, and subsurface deformations that were described in the previous paragraphs.

4. Conclusions

The fretting-corrosion behavior of the Stellite 6 cobalt-based overlay and 304 stainless steel subjected to simulated high-temperature and high-pressure PWR water conditions were systematically studied with different material combinations (C-C, C-S, and S-S). The final findings can be summarized as:
  • Material pairing is a very important consideration in the development of friction stability and fretting-corrosion damage. Self-mated contact (C-C) of cobalt-based alloys displays less friction and more consistency in friction than those of stainless steel.
  • The main damage mechanism depends on the material combination. Mechanically dominated wear with continuous plastic deformation dominated the C-C contact, but the C-S and S-S contacts exhibited increased wear–corrosion interaction with debris build-up, oxide film instability, and friction fluctuation.
  • Oxide film stability rather than oxide species determines interfacial behavior. Although similar oxide compositions were detected on different contact pairs, differences in their spatial distribution and stability led to distinct damage morphologies and friction responses.
  • Subsurface strain accommodation governs interfacial stability. Concentrated near-surface plastic deformation in the C-C contact promoted a relatively stable tribological state, while localized and discontinuous deformation in contacts involving stainless steel contributed to progressive damage and friction instability.

Author Contributions

Y.G.: Writing—original draft, Resources, Data curation. C.L.: Supervision, Methodology. Z.Z.: Writing—review & editing, Formal analysis. S.X.: Writing—review & editing, Methodology. B.Y.: Writing—review & editing, Methodology. Q.L.: Conceptualization. J.W.: Writing—review & editing. S.Z.: Writing—review & editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed at the corresponding authors.

Conflicts of Interest

All authors were employed by the company Suzhou Nuclear Power Research Institute. The company was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.

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Figure 1. Metallographic microstructure of the Stellite 6 overlay.
Figure 1. Metallographic microstructure of the Stellite 6 overlay.
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Figure 2. Specimen geometry and fretting test configuration: (a) schematic of the flat and boss specimens; (b) schematic of the specimen assembly during fretting testing.
Figure 2. Specimen geometry and fretting test configuration: (a) schematic of the flat and boss specimens; (b) schematic of the specimen assembly during fretting testing.
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Figure 3. Macroscopic morphology (a–c) and three-dimensional surface profiles (d–f) of fretting-corrosion scars for different material pairings on the flat specimens: (a,d) C–C; (b,e) C–S; (c,f) S–S.
Figure 3. Macroscopic morphology (a–c) and three-dimensional surface profiles (d–f) of fretting-corrosion scars for different material pairings on the flat specimens: (a,d) C–C; (b,e) C–S; (c,f) S–S.
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Figure 4. SEM images of fretting-corrosion scars: (a) C-C; (b) C-S; (c) S-S.
Figure 4. SEM images of fretting-corrosion scars: (a) C-C; (b) C-S; (c) S-S.
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Figure 5. EDS elemental maps of fretting-corrosion scars.
Figure 5. EDS elemental maps of fretting-corrosion scars.
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Figure 6. XPS spectra of fretting-corrosion scars: (a–c) survey spectra; (d) O1s; (e) Cr2p.
Figure 6. XPS spectra of fretting-corrosion scars: (a–c) survey spectra; (d) O1s; (e) Cr2p.
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Figure 7. EBSD IPF maps (a–c) and corresponding KAM maps (d–f) of fretting-corrosion scars for different contact pairs: (a,d) C–C; (b,e) C–S; (c,f) S–S.
Figure 7. EBSD IPF maps (a–c) and corresponding KAM maps (d–f) of fretting-corrosion scars for different contact pairs: (a,d) C–C; (b,e) C–S; (c,f) S–S.
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Figure 8. Evolution of the coefficient of friction during fretting corrosion.
Figure 8. Evolution of the coefficient of friction during fretting corrosion.
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Table 1. Chemical composition of the Stellite 6 overlay and 304 stainless steel (wt.%).
Table 1. Chemical composition of the Stellite 6 overlay and 304 stainless steel (wt.%).
Material StandardCSiMnPSCrNiFeWMoCo
Stellite 6 (measured)1.121.090.040.0027<0.000228.841.852.564.620.2659.5
RCC-M [22]0.9–1.4≤2.0≤1.0≤0.030≤0.01526.0–32.0≤3.0≤3.03.0–6.0≤1.0Remainder
304 (measured)0.0320.511.660.02<0.0119.889.72Remainder/0.06/
RCC-M [22]≤0.06≤1≤2.0≤0.04≤0.0317.0–20.09–12Remainder///
Table 2. Fretting-corrosion test groups and specimen identification.
Table 2. Fretting-corrosion test groups and specimen identification.
Material PairingSpecimen CodeSpecimen GeometrySpecimen RoleMaterial
C-CC-C1BossCounter specimenStellite 6 overlay
C-C1′FlatFixed specimenStellite 6 overlay
C-C2BossCounter specimenStellite 6 overlay
C-C2′FlatFixed specimenStellite 6 overlay
C-SC-S1BossCounter specimenStellite 6 overlay
C-S1′FlatFixed specimen304 stainless steel
C-S2BossCounter specimenStellite 6 overlay
C-S2′FlatFixed specimen304 stainless steel
S-SS-S1BossCounter specimen304 stainless steel
S-S1′FlatFixed specimen304 stainless steel
S-S2BossCounter specimen304 stainless steel
S-S2′FlatFixed specimen304 stainless steel
Table 3. Mass loss of specimens after fretting-corrosion tests under different contact pairs.
Table 3. Mass loss of specimens after fretting-corrosion tests under different contact pairs.
Specimen IDMass Before Test (mg)Mass After Test (mg)Mass Loss (mg)Specimen IDMass Before Test (mg)Mass After Test (mg)Mass Loss (mg)
C-C15465.265464.760.50C-C1′5273.235272.980.24
C-C25400.045399.710.33C-C2′5333.985333.860.12
C-S15475.495474.271.22C-S1′5166.135166.43−0.30
C-S25523.505522.620.88C-S2′5122.465122.68−0.21
S-S15118.385118.310.07S-S1′5042.625042.620.01
S-S25053.605053.460.14S-S2′5031.885031.780.10
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MDPI and ACS Style

Gui, Y.; Li, C.; Zhu, Z.; Xu, S.; Yang, B.; Li, Q.; Wan, J.; Zhang, S. Fretting-Corrosion Behavior of Stellite 6 Overlay Welded on 304 Stainless Steel in Simulated PWR Water Environment. Lubricants 2026, 14, 149. https://doi.org/10.3390/lubricants14040149

AMA Style

Gui Y, Li C, Zhu Z, Xu S, Yang B, Li Q, Wan J, Zhang S. Fretting-Corrosion Behavior of Stellite 6 Overlay Welded on 304 Stainless Steel in Simulated PWR Water Environment. Lubricants. 2026; 14(4):149. https://doi.org/10.3390/lubricants14040149

Chicago/Turabian Style

Gui, Yuanbin, Chengtao Li, Zhaoguang Zhu, Sunwu Xu, Bin Yang, Qianwu Li, Jing Wan, and Shugang Zhang. 2026. "Fretting-Corrosion Behavior of Stellite 6 Overlay Welded on 304 Stainless Steel in Simulated PWR Water Environment" Lubricants 14, no. 4: 149. https://doi.org/10.3390/lubricants14040149

APA Style

Gui, Y., Li, C., Zhu, Z., Xu, S., Yang, B., Li, Q., Wan, J., & Zhang, S. (2026). Fretting-Corrosion Behavior of Stellite 6 Overlay Welded on 304 Stainless Steel in Simulated PWR Water Environment. Lubricants, 14(4), 149. https://doi.org/10.3390/lubricants14040149

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